1. Field of the Invention
The present invention relates to a technology for storing information (or read/write control information) for use to control a write pulse when information is written on an information storage medium.
2. Description of the Related Art
On a conventional information storage medium, stored is read/write control information for use to perform a write operation.
As far as optical discs are concerned, for example, as disclosed in Japanese Patent Application Laid-Open Publication No. 2006-313621 (which will be referred to herein as “Patent Document No. 1”), so-called “disc information” (which will be abbreviated herein as “DI”), which is a piece of information including read/write control information, is stored on a Blu-ray Disc (which will be abbreviated herein as “BD”).
Specifically, in DVD-RAMs, DVD-RWs, DVD+RWs, DVD-Rs and DVD+Rs, the “physical format information” (which will be abbreviated herein as “PFI”) corresponds to the DI.
Also, even if read/write control information is already stored on an information storage medium, additional read/write control information could sometimes be written by an information storage medium recorder/player on a designated area on the information storage medium or in an internal memory of the recorder/player either in the same format as, or in a format similar to, the DI or the PFI mentioned above.
Hereinafter, the DI of a BD will be described as an example of the read/write control information.
FIG. 1 schematically illustrates the structure of a BD.
As for a single-layer medium with only one information storage layer, the only storage layer thereof has the structure shown in FIG. 1. In a multilayer medium with two or more information storage layers, on the other hand, either each or at least one of those layers has the structure shown in FIG. 1.
According to Patent Document No. 1, at least one piece of DI is stored in an area called “lead-in area 11” in the BD structure shown in FIG. 1.
Also, according to Patent Document No. 1, the DI has a size of 112 bytes and a single write pulse waveform, associated with a single layer and a single writing speed, is stored in the one piece of DI.
FIG. 2 schematically shows the makeup of the DI.
The DI consists of header information 21, read/write control information 22, and footer information 23. The read/write control information includes a disc control information part 24 including information about the type and the structure of the disc, a power information part 25 for use to control the power during reading or writing, and a write pulse information part 26 for use to control the write pulse waveform during writing.
A write pulse applied to make a recording mark on an optical disc includes power information indicating the power level of that pulse and write pulse information indicating the position and width of that pulse.
In the following description, those parameters of a write pulse will be referred to herein as a “write strategy” (which will be sometimes simply referred to herein as “WS”).
The power information of the write strategy is stored in the power information part 25 and the write pulse information is stored in the write pulse information part 26.
Hereinafter, the power information and the write pulse information will be described by reference to an exemplary write pulse waveform shown in FIG. 3.
FIG. 3 illustrates an exemplary write pulse waveform for making an 8T mark, which is eight times as long as the width T of a channel clock pulse.
In the example illustrated in FIG. 3, the power information includes pieces of information about the parameters of a write pulse in the amplitude direction, including a peak power Pw (31), a space power Ps (32), a cooling power Pc (33), and a bottom power Pb (34).
On the other hand, the write pulse information includes pieces of information about the parameters of a write pulse in the time axis direction, including a top pulse width Ttop (35), a top pulse start point dTtop (36), a multi-pulse width Tmp (37), a last pulse width 38, and a cooling pulse end point dTs (39).
These parameters are included in the DI in the format shown in FIG. 4, for example. Optionally, these pieces of read/write control information could also be written by an information storage medium recorder/player in a designated area on an information storage medium in the same format as, or in a format similar to, the one shown in FIG. 4.
Also, in some cases, an information storage medium recorder/player could store those pieces of read/write control information in its internal memory, for example.
However, as the writing speed or the storage density increases, not just the read/write control information but also the degree of thermal interference between the marks will depend more and more on the length of the space that precedes or succeeds a mark.
If a write operation were performed using the same write pulse information with respect to each of preceding or succeeding spaces that have varying lengths when such a phenomenon is observed, the length of the recording mark left would vary according to the length of its preceding or succeeding space.
Consequently, each piece of write pulse information may sometimes be defined by not only the length of each mark but also that of a space that precedes or succeeds that mark.
Recently, as the densities of information storage media have been increasing year by year, the shortest mark length of recording marks has come closer and closer to the limit of resolution that depends on the detection system.
If the information storage medium is an optical disc medium, for example, the “resolution that depends on the detection system” refers to the optical resolution to be determined by the size of a light beam spot being formed by condensing a laser beam.
Since the shortest mark length is on the verge of reaching that limit of resolution, an increase in intersymbol interference and a decrease in SNR (signal to noise ratio) have become more and more significant these days.
Hereinafter, this phenomenon will be described with an optical disc medium that uses a blue laser beam with a wavelength of 405 nm and that has a diameter of 12 cm taken as an example.
According to “Blu-ray Disc Reader” (published by Ohmsha, Ltd.), in an optical disc medium that uses a blue laser beam, the light beam spot formed by condensing a laser beam has a size of 390 nm. And if the storage capacity per storage layer is 25 GB and if RLL (1, 7) is used as a recording code, the shortest mark has a length of 149 nm.
If such an optical disc medium needs to have a storage capacity of 33.3 GB per storage layer, the shortest mark should have a length of 112 nm. And to further increase the storage capacity, the shortest mark should have an even shorter length.
Supposing the same detection system is used, if the storage capacity per storage layer is 25 GB, then a single light beam spot 51 covers 2.6 shortest marks as shown in portion (a) of FIG. 5. However, if the storage capacity per storage layer is increased to 33.3 GB, a single light beam spot 51 covers 3.5 shortest marks as shown in portion (b) of FIG. 5. Consequently, the length of each mark decreases with respect to the same size of the light beam spot formed by a detection system for the optical disc medium.
As a result, the mark/space combination to fall within the light beam spot could not just be a combination of a single mark and its preceding or succeeding space but also be a combination of multiple marks and multiple spaces.
FIG. 6 illustrates typical relations between the mark at the present time i and patterns each including multiple marks or spaces.
Portion (a) of FIG. 6 illustrates a combination of a mark at the present time i, the preceding space at a time i−1, and the succeeding space at a time i+1.
Portion (b) of FIG. 6 illustrates a combination of a mark at the present time i, the previous mark at a time i−2, and the preceding space at the time i−1.
Portion (c) of FIG. 6 illustrates a combination of a mark at the present time i, the succeeding space at the time i+1, and the next mark at a time i+2.
Portion (d) of FIG. 6 illustrates a combination of a mark at the present time i, the preceding space at the time i−1, the succeeding space at the time i+1 and the next mark at the time i+2.
And portion (e) of FIG. 6 illustrates a combination of a mark at the present time i, the previous mark at the time i−2, the preceding space at the time i−1 and the succeeding space at the time i+1.
Thus, as the storage density is increased, the write pulse information will be defined by not just a combination of a current mark and the preceding or succeeding space but also a combination of the current mark and the preceding and succeeding spaces, a combination of the current and previous marks and the preceding space, a combination of the current and next marks and the succeeding space, a combination of the current and previous marks and the preceding and succeeding spaces, and a combination of the current and next marks and the preceding and succeeding spaces as shown in FIG. 6.
Furthermore, to get a high-density write operation done, people might try to achieve high writing performance by increasing the resolution of the write pulse information.
As described above, as the storage density is increased, it is expected that the write pulse information would be expanded and the size of the read/write control information would increase.
And if the size increased too much, then such an excessive amount of information could no longer be stored in a read/write control information storage area (to store DI or PFI, for example), of which the size is determined beforehand.
Such a problem could be overcome by changing the formats of the read/write control information storage area. If the formats were changed, however, then no compatibility would be ensured with respect to media of lower orders or older generations.
Furthermore, it is not impossible to store that increased amount of information in two or more read/write control information storage areas separately. In that case, however, information would be read more slowly, and the increase in the number of DI and PFI items would leave an even narrower space in the lead-in area, which are problems.